1. Field of the Invention
The present invention relates to a signal line driving circuit and a signal line driving method, which are used in a liquid crystal display, such as an active matrix type and the like, and suitable for a small consumptive electric power.
2. Description of the Related Art
In a recent liquid crystal display, a driving circuit having a high current driving performance and a small consumptive electric power has been required in association with a larger size of a liquid crystal panel and a higher picture quality. As a display satisfying such a requisition, a liquid crystal display having a pre-charging unit is well known (for example, Japanese Laid Open Patent Application (JP-A-Heisei, 8-87248). This pre-charging unit applies a predetermined standard potential to a signal line immediately before applying a gradation signal to a pixel capacitor arranged on the liquid crystal panel. Accordingly, a load on an output unit of the driving circuit can be reduced to thereby attain an electric power saving. Also, a dispersion in the load for each TFT can be suppressed to thereby attain a stably displaying operation.
FIG. 9 is a circuit diagram showing a conventional liquid crystal display having such a pre-charging unit. It will be described below with reference to FIG. 9.
A pre-charging circuit as a pre-charging unit 25 is provided with: switches SW11, SW21, . . . , SWn1 for connecting signal lines S1, S2, . . . , Sn and output sides of a signal line driver 22, respectively; and switches SW12, SW22, . . . , SWn2 for connecting the signal lines S1, S2, . . . , Sn and a middle potential Vp, respectively. The respective switches SW11, SW12, . . . , SW1n are operated on the basis of a signal sent by a timing generator 21.
FIG. 10 is a block diagram showing an example of the signal line driving circuit in the liquid crystal display of FIG. 9. FIG. 11 is a timing chart showing the operation of the signal line driving circuit of FIG. 10. It will be described below with reference to FIGS. 9 to 11.
The signal line driver 22 is provided with: a shift register 31 operated in accordance with a clock signal CLK; a data register 32, which is controlled by an output of the shift register 31, for storing m-bit picture data in parallel; a data latch 33 for collectively transferring and storing the picture data on the basis of a control signal LP; a decoder 34 of an m-bit input; an analog switch 35 for selecting a gradation voltage from 2m voltages inputted from a gradation voltage generator 37 and outputting it; an output circuit 36 for outputting the gradation voltage outputted from the analog switch 35 to the pre-charging circuit 25; and the pre-charging circuit 25.
The switches SWn1, SWn2 are respectively turned on and off in accordance with pulse signals SP1, SP2 sent from the external timing generator 21.
As can be understood from periods T1, T2 of FIG. 11, the conventional signal line driving circuit always carries out the pre-charging operation irrespectively of the gradation voltage sent before and after a horizontal period. Such a pre-charging operation is the very effective unit in applying a gradation voltage having a different polarity such as one dot inversion drive. However, if a gradation voltage of a picture data to be next displayed is equal to a gradation voltage of a picture data before the one horizontal period or it is within a certain range of that gradation voltage, the execution of the pre-charging operation causes a voltage fluctuation in the signal line to be larger, which results in a problem that a consumptive electric power is conversely increased correspondingly to the voltage fluctuation.
By the way, especially in the field of various portable apparatuses such as a portable telephone terminal and the like, the size of the liquid crystal panel is limited because of that property. Thus, a current driving performance equal to that of a driver of a large liquid crystal panel is not required. However, the electric power saving is further desired.
It is therefore an object of the present invention to provide a liquid crystal display and a signal line driving circuit, in which a consumptive electric power is further dropped by improving the above-mentioned problems.